pub type snd_mixer_elem_callback_t = Option<unsafe extern "C" fn(elem: *mut snd_mixer_elem_t, mask: c_uint) -> c_int>;
pub enum snd_mixer_elem_callback_t { None, Some(unsafe extern "C" fn(*mut _snd_mixer_elem, u32) -> i32), }
No value.
Some value of type T.
T